docs: def normalize_url(url):
      """Return a normalized url with trailing and without leading slash.
  
       >>> normalize_url(None)
       '/'
       >>> normalize_url('/')
       '/'
       >>> normalize_url('/foo/bar')
       '/foo/bar'
       >>> normalize_url('foo/bar')
       '/foo/bar'
       >>> normalize_url('/foo/bar/')
       '/foo/bar'
      """
      if not url or len(url) == 0:
          return '/'
      if not url.startswith('/'):
          url = '/' + url
      if len(url) > 1 and url.endswith('/'):
          url = url[0:len(url) - 1]
      return url